On separate ends of the scale, an action thriller about North Korean terrorists taking over the White House, and a prehistoric family adventure have surprised film distributors by scoring high with audiences.

The caveman comedy The Croods left an indelible mark on the wall, opening at No.1 with an estimated $US44.7 million ($A43.04 million).

The 3D adventure from DreamWorks Animation and 20th Century Fox features a voice cast including Nicolas Cage, Emma Stone and Catherine Keener. They play a prehistoric family encountering danger and strange new creatures when they're forced to find a new cave.

The Croods has now made $US108 million worldwide, also opening this weekend in Russia, the United Kingdom, Germany and Spain.

In the United States, it's the only game in town as far as animated films for the whole family, and it will continue to be so as kids head out of school for (the northern) spring break over the next couple of weeks.

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"It's a terrific crowd-pleaser, it got an 'A' CinemaScore and an 'A-plus' with audiences under 18, which leads me to believe a lot of kids loved the movie," said Anne Globe, chief marketing officer for DreamWorks Animation.

Chris Aronson, president of domestic distribution for 20th Century Fox, said The Croods had a stronger opening than the $US40 million the studio had projected.

"To come in at the $45 million mark, ahead of How to Train Your Dragon, which was another terrific movie from DreamWorks Animation, is a great start to the spring holiday," Aronson said.

Olympus Has Fallen also opened higher than expectations - much higher - given that FilmDistrict figured it would end up somewhere in the under-$US20 million range, said president of distribution Jim Orr.

This is by far the biggest debut for the independent distributor, which was just founded in September 2010; the previous best was the $US14.3 million the Red Dawn remake made over last year's US Thanksgiving holiday weekend.

"Millennium Films and Antoine Fuqua delivered a brilliant, action-packed, serious thriller with an all-star cast led by Gerard Butler, and the word of mouth seems to be terrific," said Orr. "It not only exceeded all our pre-weekend estimates, as the weekend has gone on, it's gotten better.

"This is the first action thriller in a while that's gotten an 'A-minus' CinemaScore, so it's obvious that people are talking about it and enjoying it."

This one-two punch of Croods and Olympus - two movies that appealed to two very different audiences - was much-needed at the box office, which is down 13 per cent from the same period last year, said Paul Dergarabedian, box office analyst for Hollywood.com.

"I was really surprised by Olympus. This year, the R-rated action films have all fallen flat," Dergarabedian said, including The Last Stand and Parker. "The marketing was good. Gerard Butler - he's the real deal, he looks the part and everything.

"And the theme of the movie, the fact that the president is under siege - it worked on Air Force One. There's something about that theme that works for audiences."

But this weekend's haul is down 34 per cent from the same weekend a year ago, when the juggernaut of The Hunger Games debuted. It made a whopping $US152.5 million in its opening, which is more than all the films in theatres combined will have made this weekend.

Among the other new films this weekend, the Tina Fey-Paul Rudd college comedy Admission from Focus Features opened in fifth place with just $US6.4 million.

But the buzzed-about Spring Breakers from A24 Films, featuring Selena Gomez and Vanessa Hudgens as hard-partying college girls, did well in its first nationwide expansion in the US. It made $US5 million on 1104 screens for a total of $US5.4 million over the past two weeks.

Nothing really huge is on the horizon to give the box office a boost until Iron Man 3 kicks off the US summer movie season on May 3, Dergarabedian said. But the strong showing for Olympus Has Fallen could bode well for the sequel G.I. Joe: Retaliation next weekend.